Holograms and diffraction patterns have gained acceptance in recent years as anti-counterfeiting and anti-tamper devices. Such devices are typically made by embossing a plastics film with an optically diffracting pattern and providing a metallic surface which follows the finely embossed structure such that on viewing the optical effect can be seen.
Often the holographic image is made to reconstruct in white light. The reflective metallic structure is generally viewed through the transparent plastic. Alternatively it is possible to obtain an holographic effect which may be brighter by viewing directly the reflective metal coating on the embossed plastic, provided that the metallic coating maintains the diffracting surface's profile.
It is possible for the polymeric surface of these mass produced replicas to be made by embossing from a nickel or hard plastic master directly into a thermo-plastic film, or by causing reactive entities to polymerise or crosslink under the influence of radiation such as electron beam radiation or ultraviolet radiation while in contact with the master. In both cases the surface accurately takes the shape of the metallic or other mould which holds the fine line structure. Unmetallised polymeric surfaces are then typically metallised with a thin coating of, for example, aluminium.
EP-A-0201323 describes a transparent hologram having an embossed layer of transparent plastic, coated with a thin transparent layer which has a different refractive index to enhance the quality of the hologram. The hologram so formed may be positioned on a substrate carrying printed information which will still be visible through the hologram.
Holograms have seen increasing use for anticounterfeiting, antiforgery, antitamper and authentication purposes over recent years and that success continues to be enjoyed. Authenticatable items such as credit cards will generally have discrete hologram labels stamped on under heat and pressure from a carrier film but this is a time consuming process. The size of the areas which can be applied is also limited. Additionally hot stamping is only suitable for smooth substrates as coarse substrates may cause texturing of the hologram. This may present both undesirable aesthetic qualities and difficulties in machine reading of encoded holograms. Overprinting such labels is often difficult.
The above cards generally employ discrete holographic images such as of a bird. These images must be placed in register within the label and the label must be in register with the printing on the remainder of the article. It is however possible in some applications where image-to-label registration is not necessary to provide a substantially continuous pattern of repeating holographic images of small size. The print registered stamping of this type of pattern does not require precise image-to- label registration as whatever area is stamped will hold a number of the small images. (This is sometimes called a "wallpaper" or "generic" design.)